In the present description, by «movable terrestrial machinery equipment», it is referred to any self-propelled or trailed machinery equipment moving on the ground. In turn, the expression «variable-track» refers to any machinery equipment equipped with a device adapted for modifying the distance between two wheels of the same axle.
Conventionally, such a machinery equipment comprises a chassis resting on a front axle and on a rear axle, each axle receiving at its ends a knuckle on which the hub of a wheel is engaged. An axle may consist only of a carrier axle or it may be a drive axle.
In general, the displacement of a vehicle is achieved by the rotation of at least one axle, driven in rotation directly or indirectly by an output shaft of a motor, for example a heat engine or an electric motor. In turn, the direction of displacement is controlled by a steering control device acting on the front and/or rear wheels.
In the case where the front wheels are steerable, each front wheel is secured to a steering knuckle assembly which is, in turn, secured to a steering tie rod movable in rotation about a substantially vertical axis.
The rotation of the steering tie rods is controlled from the driver's cab by the driver who acts on a steering wheel.
A steering control device known in the prior art comprises a steering wheel secured to a steering column at the end of which is mounted a pinion meshing with a rack, secured, at each one of its ends, to the steering tie rods.
There is also known a steering control device according to which the steering tie rods are, on the one hand, secured to a steering knuckle assembly supporting the wheel, and on the other hand, pivotally mounted at their free end on a rod of a hydraulic or pneumatic double-acting cylinder, the displacement of the rod of the cylinder being ensured by the introduction of a fluid or a gas at either side of the piston.
The introduction of the fluid into the cylinder is controlled from the driver's cab by the rotation of the steering wheel, engaged with a hydraulic pump and with a system of appropriate valves known by those skilled in the art.
However, regardless of the retained embodiment of the steering control device, during a turn, the steered wheels of the vehicle do not cover the same distance. Hence, it is necessary that, during a turn, the steered wheels do not remain parallel to each other. For this purpose, it is necessary to provide for distinct angles of rotation for the right front wheel and for the left front wheel, in the case where the steered wheels of the vehicle are the front wheels.
If this condition is not met, a wheels shifting phenomenon happens, which phenomenon should be avoided, on the one hand, in order not to undermine the safety of the persons on-board (the shifting results in an excessive slipping of the wheels on the ground) and, on the other hand, in order to prevent an excessive and premature wearing of the tires.
In order to avoid this phenomenon, the sizing of the different members constituting the steering control device is typically carried out based on the Ackermann steering geometry, well known by those skilled in the art.
FIG. 1 schematically represents a machinery equipment 1 comprising a front axle 2 at the ends of which are mounted two steerable front wheels 3G, 3D, and a rear axle 5 at the ends of which are mounted two rear wheels 7G, 7D.
FIG. 2 illustrates the Ackermann steering geometry applied to the represented machinery equipment 1, in a cornering situation.
The non-slip condition is met when, during a turn, the lines of contact with the ground of the four wheels are tangent to the circle defined by the turn.
In other words, the non-slip condition is met when the four wheels have a common center of rotation (commonly called «ICR», standing for «Instantaneous Center of Rotation»), corresponding to a sufficient opening angle α between the wheels. If the front wheels are the only steerable wheels, the common center of rotation is then necessarily located on the axis of the rear axle.
Thus, as is represented, for a right turn, the Ackermann steering geometry assesses that the non-slip condition is met if the line (dD) perpendicular to the line of contact of the right front wheel 3D with the ground and the line (dG) perpendicular to the line of contact of the left front wheel 3G with the ground intersect on the line (d5), which lines links the two points of contact of the rear wheels 7G and 7D with the ground. The point of intersection of these two lines is also called the Ackermann point.
Non-slipping in turns is achieved thanks to a specific sizing of the steering tie rods 9G, 9D and a control arm 11, schematically represented in FIG. 2 for a better understanding of the system. For this purpose, as illustrated in FIG. 3, the Ackermann principle requires the steering tie rods 9G, 9D to be positioned so that their longitudinal axes (d9G), (d9D) intersect in the vertical plane passing through the axis (d5) of the rear axle 5, at the middle of the axle, when the vehicle is in a rectilinear displacement.
Some vehicles, such as, for example, sprayer-type agricultural machinery equipment's or straddle machinery equipment's are so-called «variable-track» vehicles, that is to say that the distance between two wheels of the same axle is variable between an enlarged position, allowing for example for a better stability on land when the vehicle is used for land treatment purposes for example, and a retracted position, allowing in particular the vehicle to adapt to the widths of all tracks of the roadway.
In a known manner, switching between the positions is generally carried out when the vehicle is on the land parcel to be treated. For this purpose, the vehicle is generally fitted with a hydraulic device adapted for activating at least one cylinder the attachment point of which is secured to the chassis of the machinery equipment, and the end of the movable rod of which is secured to a telescopic axle or directly to the wheel.
FIG. 4 illustrates the Ackermann steering geometry applied to the machinery equipment 1 in the enlarged position, in a cornering situation.
For an inclination of the front wheels 3G, 3D identical to that illustrated in FIG. 2 when the machinery equipment is in the retracted position, the left front wheel 3G and the right front wheel 3D have each a center of rotation ICR3G, ICR3D distinct from each other if the lines (dD) and (dG) intersect with the axis (d5) of the rear axle 5.
This results in a shifting phenomenon the drawbacks of which have been described before.
The Ackermann steering geometry applied to a four-wheel-steered machinery equipment, in a cornering situation, is now represented in FIG. 5.
A four-wheel-steered machinery equipment allows for a larger steering angle in comparison with a two-wheel-steered machinery equipment. This arrangement is particularly advantageous for agricultural machinery equipment's when performing relatively tight turns, for example on headlands.
In a cornering situation, pivoting of the left rear wheel 7G, respectively of the right rear wheel 7D, corresponds to the symmetric of the position of the left front wheel 3G, respectively of the right front wheel 3D, with respect to the midplane of the front and rear axles.
As before, the non-slip condition is met if, during a turn, the lines of contact of the four wheels with the ground are tangent to the circle defined by the turn. In other words, the non-slip condition is met if the four wheels have a common center of rotation (ICR), corresponding to a sufficient opening angle αAV and αAR between the wheels.
Typically, if the front and rear wheels are all steerable, the opening angle αAV between the right and left front wheels, and the angle αAR between the right and left rear wheels, are sufficient if each of these angles is substantially equal to half the opening angle α obtained if the machinery equipment comprises only but two steerable wheels.
If the front wheels and the rear wheels are all steerable, the common center of rotation is located in the transverse midplane 10 of the machinery equipment.
By «transverse midplane», it is referred to the plane perpendicular to the middle axis of the machinery equipment, that is to say perpendicular to the axis symbolizing the direction of displacement of the machinery equipment in a rectilinear displacement, and substantially equidistant from the front axle 2 and from the rear axle 5.
Thus, as is represented, for a right turn, the Ackermann steering geometry assesses that the non-slip condition is met if the lines perpendicular to the lines of contact of the four steered wheels with the ground intersect with the line of intersection between the ground and the transverse midplane 10 of the chassis.
As before, in the case of a variable-track vehicle such as, for example, a sprayer-type agricultural machinery equipment or a straddle machinery equipment, the distance between two wheels of the same axle is variable between an enlarged position and a retracted position.
FIG. 6 illustrates the Ackermann steering geometry applied to the four-wheel-steered machinery equipment, in the enlarged position and in a cornering situation.
For a pivoting of the wheels 3G, 3D, 7G, 7D, identical to that illustrated in FIG. 5 when the machinery equipment is in the retracted position, each steered wheel possesses its own instantaneous center of rotation, distinct from the transverse midplane 10, which results in a shifting phenomenon, in the same manner as for a two-wheel-steered machinery equipment, with the generation of the aforementioned drawbacks.